Last month, I spent some time addressing the issue of clean glass on exposure equipment. Here, I continue my look at screen exposure by focusing on the vacuum blanket and the vacuum drawdown process.

Few screen-printing topics seem more mundane than vacuum blankets. Yet, when you really look closely at this component of the screenmaking process and consider all of the implications it has on stencil and print quality, you begin to appreciate why this subject deserves to be addressed. Most notably, you need to be aware of vacuum drawdown because it directly affects image definition, acutance (edge sharpness), tone reproduction, moir&eacute; formation, and registration.

Function of the vacuum blanket

The fundamental purpose of the vacuum blanket is to hold the emulsion of the film positive in intimate contact with the emulsion of the coated screen. Anything less than intimate contact will cause undesirable light scatter (halation) that will result in undercutting and image degradation. The question is, how do you know that the vacuum is uniform and complete?

This is not an easy question to answer. Readings on gauges can be deceptive. I am often told by printers that they are achieving 26 in. of vacuum or some similarly high value. Is this good or bad? How do you know what is really happening? The answers to these questions are really quite relative.

In the first case, the pressure indicated on the gauge may or may not be reflective of the actual pressure on the screen where it and the film positive make contact. Most likely, the reading reflects the vacuum as measured at the vacuum inlet where it meets the glass. It is entirely too common for the inlet to seal at this location prematurely, resulting in localized high vacuum and no vacuum in the screen area. This obviously creates some big problems. Chiefly, the contact between the positive and the emulsion is incomplete, undercutting results, and the image is compromised.